The present invention relates to a mass spectrometer and a method of mass spectrometry.
It is a common requirement in a mass spectrometer for ions to be transferred through a region maintained at an intermediate pressure i.e. at a pressure wherein collisions between ions and gas molecules are likely to occur as ions transit through an ion guide. Ions may need to be transported, for example, from an ionisation region which is maintained at a relatively high pressure to a mass analyser which is maintained at a relatively low pressure. It is known to use a radio frequency (RF) transport ion guide operating at an intermediate pressure of around 10−3-101 mbar to transport ions through a region maintained at an intermediate pressure. It is also well known that the time averaged force on a charged particle or ion due to an AC inhomogeneous electric field is such as to accelerate the charged particle or ion to a region where the electric field is weaker. A minimum in the electric field is commonly referred to as a pseudo-potential well or valley. RF ion guides are designed to exploit this phenomenon by causing a pseudo-potential well to be formed along the central axis of the ion guide so that ions are confined radially within the ion guide.
It is known to use an RF ion guide to confine ions radially and to subject the ions to Collision Induced Dissociation or fragmentation within the ion guide. Fragmentation of ions is typically carried out at pressures in the range 10−3-10−1 mbar either within an RF ion guide or within a dedicated gas collision cell.
It is also known to use an RF ion guide to confine ions radially within an ion mobility separator or spectrometer. Ion mobility separation may be carried out at atmospheric pressure or at pressures in the range 10−1-101 mbar.
Different forms of RF ion guide are known including a multi-pole rod set ion guide and a ring stack or ion tunnel ion guide. A ring stack or ion tunnel ion guide comprises a stacked ring electrode set wherein opposite phases of an RF voltage are applied to adjacent electrodes. A pseudo-potential well is formed along the central axis of the ion guide so that ions are confined radially within the ion guide. The ion guide has a relatively high transmission efficiency.
An RF ion guide is disclosed in US 2005/0253064 wherein an RF voltage is applied to an elongated rod set in order to confine ions radially within the ion guide. A static axial electric field is arranged to propel ions along the axis of the ion guide. An RF axial electric field is also arranged at the exit of the ion guide. The RF axial electric field generates an axial pseudo-potential barrier which acts as a barrier to ions. The magnitude of the pseudo-potential barrier is inversely dependent upon the mass to charge ratio of the ions. Therefore, ions having a relatively low mass to charge ratio will experience a pseudo-potential barrier which has a relatively large amplitude. The pseudo-potential barrier counteracts the effect of the static axial field for ions having relatively low mass to charge ratios but does not counteract the effect of the static axial field upon ions having relatively high mass to charge ratios. Accordingly, ions having relatively high mass to charge ratios are ejected from the ion guide. Ions may be manipulated within the ion guide or may be mass selectively ejected by adjusting the amplitude of the static or oscillating electric fields.
The known ion guide has a well-defined radial stability condition for ions having a particular mass to charge ratio. This is determined by the approximately quadratic nature of the radial potential which is maintained. Therefore, disadvantageously, if the oscillating electric field along the axis of the ion guide is changed in any way then this may cause undesired radial instabilities and/or resonance effects which may result in ions being lost to the system.
It is therefore desired to provide an improved ion guide or mass analyser.